\begin{table}
  \centering
  \vspace{1ex}
  \begin{tabular}{|c| p{0.4 cm}| p{0.4 cm}| p{0.4cm}| c | c | c |}
    \hline
    \multirow{3}{*}{
    \parbox[c]{.2\linewidth}{\centering Voltage vectors}}
    & \multicolumn{3}{l|}{Switching vectors }
    & \multicolumn{3}{c|}{Output voltage} \bigstrut\\ \cline{2-7}
    & a & b & c & Va & Vb & Vc \bigstrut \\ \hline
    \rule{0pt}{4ex}
    V0& 0 & 0 & 0 & 0 & 0 & 0 \\ \hline
    \rule{0pt}{4ex}
    V1& 1 & 0 & 0 & 2Udc/3 & -Udc/3 & -Udc/3 \\ \hline
    \rule{0pt}{4ex}
    V2& 1 & 1 & 0 & Udc/3 & Udc/3 & -2Udc/3 \\ \hline
    \rule{0pt}{4ex}
    V3& 0& 1 & 0 & -Udc/3 & 2Udc/3 & -Udc/3 \\ \hline
    \rule{0pt}{4ex}
    V4& 0 & 1 & 1 & -2Udc/3 &  Udc/3 &  Udc/3 \\ \hline
    \rule{0pt}{4ex}
    V5& 0 & 0 & 1 & -Udc/3 &  -Udc/3 &  2Udc/3 \\ \hline
    \rule{0pt}{4ex}
    V6& 1 & 0 & 1 & Udc/3 &  -2Udc/3 &  Udc/3 \\ \hline
    \rule{0pt}{4ex}
    V7& 1 & 1 & 1 & 0 & 0 &  0\\ \hline
  \end{tabular}
\end{table}
